Process of making paper pulps



April 1, 1952 D. M. SUTHERLAND PROCESS OF MAKING PAPER PULPS Filed July 24, 1947 .mi ww n Ne N E R VZ w mw H mm .w

Patentedl pr. 1, 1952 PROCESSV OF MAKING PAPER PULPS Daniel Manson Sutherland, Morrisville, Pa., as-

signor to Lionel M. Sutherland and Douglas G.

Sutherland, trustees Application July 24, 1947, Serial No. 763,318

Claims.

This invention relates to a process of making paper pulps from wood and particularly to a process of making high-yield pulp.

In the papermaking art it has heretofore been proposed to increase the yield of wood pulp by decreasing the degree of chemical action taking place in the digester. However, other difficulties encountered in the subsequent use of such high yield pulps have led to an effort in the art to avoid'rather than to encourage their production. The present invention is directed to the production of an improved paper pulp through a process which includes limiting the chemical action taking place in the digester below that normally employed, discharging the pulp thusdigested into a blow tank and there reducing and controlling its consistency, and a special reiining step in which the pulp, while still suspended in the hot cooking liquor, is subjected to the action of a renner prior to washing or other removal of the hot cooking liquor from the pulp. The arrangement of suitable apparatus for carrying outthis process is shown diagrammatically in the drawing in which there is shown a chip bin IU adapted to feed wood chips to the digester II. After a controlled heat treatment under pressure in such digester, as described at a later point in this specication, blow-off valve 8 is opened and the pulp is transferred to blow tank I2 which has suflicient capacity to handle the total output of digester II and three additional digesters (not shown) arranged in parallel. This makes it possible to maintain continuous operation in the blow tank. An agitator 9 is provided in the lower portion of the blow tank I2 and is continuously rotated through the action of motor I3. The hot pulp, together with the associated hot cooking liquor, coming from the digesters is diluted in the lblow tank with hot brown liquor from washers 2l and 28. This brown liquor is a diluted black liquor formerly associat- 'ed with the pulp and is produced at the washers by washing the pulp which comes from the reiiner chest. The hot pulp in the blow tank is thinned by the addition of such brown liquor to a controlled consistency as later described and continuously withdrawn from the blow tank I2 to the magnetic separator I5 and Jordan I5 and thence to a number of renners I'l, I8 and I9 where the pulp is refined while still hot. Pump I4 is provided for maintaining the ow of stock. The refiners are preferably of the disc type such as shown in my U. S. Patents No. 2,035,994 and No. 2,156,321 and are arranged to operate in parallel. Motors 5, 6 and 'l are provided for driving the rotating discs of refiners I1, I8 and I9. The refined discs are of a design adapted to separate the liber bundles without producing fines but still reducing the amount. of screen rejects to less than 3% of the air dried pulp. The quantity of pulp passing to each disc renner is` controlled by gate valves 20, 2| and 22. A return line 23 is provided for carrying excess hot pulp and hot cooking liquor back to the blow tank I2 and a gate valve 2d is provided, as shown, for controlling the rate of llow in the return line.

Following the rening step the pulp is transferred to a renner chest 25 from which it is pumped by pump 26 to washers 21 and 28 which may be of the vacuum type operating in parallel. Following the washing step the pulp is transfel-red to a dump chest 29 and ultimately to a rough chest 39 through the action of pump 3|. The rough chest is a tank adapted to collect occel-sional overflow from the head box and is designed to mix this overflow with washed pulp from the dump chest, thereby returning said overflow to the head box. Hot brown liquor from washers 21 and 2B is returned to blow tank I2 through a consistency regulator shown diagrammatically at 32 which includes a regulator valve 33 controlling the iiow. Valve 33 is opened or closed in response to changes in the agitator load applied to motor I3 which is mounted on a bearing swivel base (not shown). A change in the agitator load causes motor I3 to turn on its axis in the opposite direction and this results in a lifting or lowering of regulating weights 34 and a corresponding adjustment of the flow through valve 33. a

From the rough chest 39 the stock is transferred by pump 35 to headbox 36 which is provided with an overflow compartment 31 and an `overow line 38 feeding back to rough chest 30. From headbox 36 the stock is passed to fine screens 39 and thence through lter 40 and repulper 4i to lter chest 42. The rejects from the ne screens39 are returned through' line 43 to blow tank I2' thus forming a closed systemA and eliminating separate equipment for refining the rejects.

Having described a suitable arrangement of apparatus for carrying out the process of the present invention, I will now set forth preferred conditions of operation for carrying my process into effect. The description which follows is; ap plicable particularly to what is known in the art as the alkaline process and particularly to the sulfate process. It will be understood, however. that by selecting suitable comparable conditions the process of the present invention may be applied to the making of paper pulp Iby the soda process or by the neutral sulte process.

At the start of the operation the digesters are charged with wood chips of the usual size and length and the coversl bolted on. The charge' of alkali used in the digesters is selected in the range between 400 and 450 pounds of alkali (measured as NazO) per ton of air dry pulp. By means of steam the temperature of the digester charge is raised to about 340 F. at a pressure of about 110 pounds per square inch in a time period of about one hour and forty-five minutes. The cook is then held at this maximum temperature and pressure for a period of about fiftyiive minutes after which it is transferred from the digester to the blow tank.

Alternatively, a lower concentration of alkali and a longer digesting time may be employed. It is also possible to use a lower temperature and more time. In all such adjustments or variations of the three variables controlling the degree of Acooking it is only necessary to achieve the desired limited action while producing a cook suiciently hot to have the desired temperature when it ultimately reaches the refiners in its diluted form.

The pulp and its associated black liquor is blown from the digester at about 340 F. into a blow tank. However, this temperature drops immediately to about 220 F. in the flashing-ofi which occurs in the transition from 105 pounds per square inch to atmospheric pressure. The consistency is then reduced from about 12 to 15% fiber solids to about 3.25% fiber solids using hot brown liquor from the washers for the dilution. This dilution, coupled with heat radiation losses, further reduces the temperature to about 200 F. to 210 F. depending upon the temperature of the brown liquor coming from the washers which is normally about 160 F. to 180 F. At a controlled consistency of 3.25% fiber solids the pulp is then passed through a separator and, where necessary, a Jordan or other chip breaker to the refiners. This is carried on as a continuous flow process and the temperature of the pulp as it comes to the renner is about 180 F. to 210 F.

While it is possible to use various types of precision refiners, provided the clearance is suiiciently close and can be so maintained, the type which I have found most practical and emcient is made up of one stationary and onerotating disc driven at about 450 R. P. M. by a 350 horse power motor. It is possible to use either 42 inch or 48 inch discs but in either case the disc pattern and construction should be such as to provide a rubbing out of the'iibers and fiber bundles suspended in the hot alkaline liquor. Nickelchromium steel discs have proven very satisfactory for this purpose. The degree of refining is controlled by regulating the back pressure of pulp in the line and by properly setting the clearance between the reflner discs. An ammeter (not shown) on each rener is employed to measure the current being used. As the discs are set closer the power input and the back pressure increase. I have found that optimum results are obtained by operating the renners in such a way that each motor is delivering from 140 to 175 horsepower. This is accomplished by properly adjusting the disc setting and the inlet pressure to give the requisite current indication at the applied voltage.

The wood chips introduced into the digester consist of fibers which are primarily composed of cellulose. These fibers are held together by non-cellulosic encrustants of which the principal constituent is lignin. The process of the present invention is directed to weakening the encrustant bond holding such fibers together without resorting to treatment which destroys or degrades any substantial part of the cellulose fibers or fiber 4 bundles. This is accomplished by a treatment in the digester which is sufficient to weaken the bond but is not vigorous enough either to complete the required fiber separation or to bring about degradation of the cellulose. It is my belief that this limited treatment in the digester has a softening effect on the lignin and other encrustants but does not result in any substantial separation of the liber bundles. Separation of the fiber bundles is accomplished in a separate step by rubbing such bundles out while hot and alkaline in a suitable rener. In this connection it is important to note that the necessary separation of the fibers and liber bundles in the refiner cannot be satisfactorily accomplished if the pulp coming from the digester is allowed to cool. Where this is attempted a physical degradation of the fibers takes place and the reflner cannot properly handle the pulp in the substantially nonplastic form which results from cooling. For a. number of reasons it is not possible to pass the pulp directly from the digester into the rener. Such a procedure is not suitable in the practice of the present invention due to the fact that the fiber bundle separation carried out in the reiiner requires a very close and accurately controlled clearance between the rennerl discs and it is not possible to operate the renner discs at such close clearances at the consistency of the pulp coming directly from the digester. Moreoven tramp metals which are always present in the digester pulp, even where magnetic separators are used on the incoming chips, would quickly destroy the effectiveness of the reflner discs and the necessary ber bundle separation would not be obtained. A V

The pulp produced by the process of the present invention is entirely diiierent both in composition and characteristics'from any pulp produced using conventional processes of cooking, washing, screening and then refining. This will be apparent from the characteristics set forth in the following tables covering characteristics of the pulp and also unbleached kraft paper made from such pulp for test purposes.

Pulp as it comes from the refiners:

TAPII BEATER TEST, 6500 GRAMS ON BED PLATE LEVER Beating Freeness Burst Tear Length Breaking (mins.) (ce.) Bulk Factor Factor (Metres) ltrggt ON FREENESS BASIS 500 1. 55 89 V. 9U 11, 9U() 3. 5

The results indicated in the above tables appear to be attributable to the combined effect of preventing certain changes and losses normally occurring in the digester, holding the pulp so produced for a short time in suspension in the blow tank While adjusting its consistency, and subsequently refining the diluted pulp while still suspended in the hot cooking liquor.

Without intending to restrict my invention by suggesting' an explanation for these unique results. it is my belief that the customary process of digesting, Washing and then refining requires such an active treatment in the digester as to result in considerable damage or degradation of the cellulose fibers even though the aim is to affect only constituents of the Wood other than the cellulose. The process of the present invention avoids such damage in the digester by limiting the action taking place there and provides a later treatment (agitation in the blow tank followed by hot rening) which completes the desired separation without the degradation damage which would accompany further alkali treatment in the digester.

One of the reasons for the failure of prior attempts to produce satisfactory high-yield pulps has been that excessivefoaming has occurred during the refining stage. I have found that such foaming does not occur if the temperature of the pulp suspension is kept above 150 F. at the reners. I attribute this improvement to the fact that above 150 F. a sufiicient amount of steam is flashed oi from the pulp suspension to prevent air infiltration.

Furthermore, in the prior art it is necessary to use knotters and pulp screens to remove the knots and fiber bundles. In my invention the use of knotters is avoided and the amount of screen rejects is reduced below 3%. Again in the prior art it is necessary to have some methods of refining the knots and screen rejects. In some cases the knots and large uncooked pieces of wood are returned to the digester for recooking which lowers the capacity or production of the digesters to the extent that knots are returned. The screen rejects of the prior art are sent to some type of rening equipment such as attrition mills at a temperature of 70-90 F. for further reiinement and are then returned to the pulp screens. On the other hand, my invention eliminates separate refiners for screen rejects and thus reduces the cost of production.

Having thus described my invention, I claim:

1. In a process for making paper pulp from wood, the steps including heating pieces of Wood immersed in alkaline cooking liquor to a temperature of about 340 F. at a pressure ofabout 110 pounds per square inch for about 55 minutes; then blowing the cooked woodand the cooking liquor to a blow tank and diluting the cook with hot alkaline wash liquor at about to 180 F. in said blow tank to a consistency of about 3.25% ber solids while maintaining the temperature at about 200 F.; and then refining the diluted cook at about 180 F. While suspended in the hot alkaline liquor. l

2. In a process for making paper pulp from wood, the steps of subjecting pieces of Wood to the digesting action of hot alkaline cooking liquor at about 340 F. and at a pressure of about 110 pounds per square inch for a limited time interval suflicient only to soften the encrustant bond holding the cellulose fibers together without substantially separating the fiber bundles; blowing the limited cook thus obtained to a blow tank at a substantially atmospheric pressure; then diluting the cook to a refining consistency by addition of hot alkaline wash liquor at about 160 to 180 F.; refining the hot diluted pulp suspen: sion thus obtained at about 180 to 210 F.; and then Washing the refined pulp while hot, thereby obtaining the hotalkaline Wash liquor used in diluting said cook.

3. The process of claim 2 further characterized by the step of agitating said limited cook during the addition of said hot alkaline wash liquor.

4. The process of claim 2 further characterized by maintaining the temperature of said limited cook in the temperature range from about 200 to about 220 F. during the addition of said het alkaline wash liquor.

5. The process of claim 2 further characterized by the step of removing tramp metals from said limited cook after the addition of said hot alkaline wash liquor and before said refining step.

DANIEL MANSON SUTHERLAND.

REFERENCES CITED The following references are of record in the iile of this patent:

UNITED STATES PATENTS Number Name Date 151,662 Herron June 2, 1874 804,313 Handford Nov. 14, 1905 1,052,675 Loomis Feb. 11, 1913 1,721,376 De La Roza July 16, 1929 1,896,344 Benson Feb. 7, 1933 1,936,697 Traquair Nov.` 28, 1933 1,982,130 Wollenberg Nov. 27, 1934 2,007,024 Richter July 2, 1935 2,018,937V Wells Oct. 29, 1935 2,035,447 Altick Mar. 31, 1936 2,035,994 Sutherland Mar. 31, 1936 2,190,193 Richter Feb. 18, 1940 2,265,622 Basler Dec. 9, 1941 2,387,429 Cate Oct. 23, 1945 FOREIGN PATENTS Number Country Date 17,342 Australia of 1934 OTHER REFERENCES Modern Pulp and Paper Making, by Witham (2nd edition, 1942), pages 172 and 176. 

1. IN A PROCESS FOR MAKING PAPER PULP FROM WOOD, THE STEPS INCLUDING HEATING PIECES OF WOOD IMMERSED IN ALKALINE COOKING LIQUID TO A TEMPERATURE OF ABOUT 540* F. AT A PRESSURE OF ABOUT 110 POUNDS PER SQUARE INCH FOR ABOUT 55 MINUTES; THEN BLOWING THE COOKED WOOD AND THE COOKING LIQUOR TO A BLOW TANK AND DILUTING THE COOK WITH HOT ALKALINE WASH LIQUOR AT ABOUT 160* TO 180* F. IN SAID BLOW TANK TO A CONSISTENCY OF ABOUT 3.25% FIBER SOLIDS WHILE MAINTAINING THE TEMPEATURE AT ABOUT 200* F.; AND THEN REFINING THE DILUTED COOK AT ABOUT 180* F. WHILE SUSPENDED IN THE HOT ALKALINE LIQUOR. 